So you're coming at this I think from the wrong angle. You seem to me to be anachronistically inserting modern, or at least early-modern, systems of taxation and economic development into the Roman state, when in fact the Roman economy did not behave the same way. For example, your first and second points both seem to suggest a per capita tax, when there was in fact not really such thing at Rome--Latin doesn't even have a word for taxes as a whole, and there definitely wasn't a concept of an individual income tax scaling to the amount of money you make like we have now. Roman law did not have a systematic method of approaching taxes, and the various duties and fees placed on particular groups or goods were very much individual legal affairs. Two major sources of income existed in the Roman state, the vectigalia and the tributum. Neither of these are income taxes and neither of them are (generally) levied per capita. The vectigals were essentially import duties and state rents--its a rather large category, and the vectigals were often used as a synonym for the state's revenue as a whole, even though the majority of state funding was probably not by this means. The vectigals included import taxes on goods, what could be considered sales taxes on a very few types of goods (the only one I know about, the 1% tax on slave auctions, was abolished in Italy under Caligula), and taxes on certain types of transactions (such as the 5% tax on the property enclosed in a citizen's will). But the vectigal was not something that you could pay just by existing, like an income tax, you had to actually be engaging in the particular transactions and movement of goods that various vectigals taxed.

The main source of income for the state as a whole was probably actually the tributum, at least up until a certain point. Originally this had been a sort of property tax on Roman citizens but from 167, B.C. on citizens were exempt from it. From then on tribute was paid only by non-citizens--that is, provincials, except for the inhabitants of coloniae, free cities, and other individuals who possessed the ius Italicum. Provincial tribute came in two forms, the tributum soli and the tributum capitis. The tributum soli was basically the old property tax, now levied on provincials. Depending on the province it might be paid either in cash or in grain and the rates differed greatly--the Sicilians, for example, paid one-tenth of their grain harvest to the city, whereas Iberia paid a fixed sum, which might be changed from year to year but was not tied to the actual production of the province. The tributum capitis is a bit harder. It was in theory a per capita poll-tax, but as we'll see the actual methods of tax-collection meant it need not have been paid individually. In the Principate it seems to have been universal in all the provinces, but it's generally not attested before that. Before the Principate it seems that only Africa had it, as Appian says it was imposed on the defeated in 146. It was levied on provincials when they came of age, but it was certainly not an income tax, being levied at a fixed rate no matter the actual assets of the individual.

So we see that already the Roman tax system was very different. In particular revenue was generally accrued not individually but according to much larger divisions--the city, region, and province paid taxes, which were raised from the individual but not paid directly by them (except in the case of some of the vectigals and at least in theory the tributum capitis). The Roman method of tax-collection supports this and made it possible. You mention tax-collectors in your second post but seem to think that disguising one's wealth would've made any difference. It didn't--there was no scaling income tax. Systems of tax collection are complicated, but in general the Roman state did not collect taxes directly, instead selling collection contracts to private companies. In the Republic publican contracts in the provinces (that is, collecting tribute--the vectigals were also collected by publicans but since they required certain economic activities to be made contracts could not be sold up front) worked essentially by auction. A publican or his company would bid a certain amount to collect from the provinces, and the highest bidder would front the money to the state and then would collect taxes to make up for whatever he just paid. Anything extra he collected was his to keep. This system was therefore pretty much entirely in the hands of the publicans, who were usually very exploitative. In the Principate the publicans, who had been much weakened by the confiscation of their wealth in the civil wars, were much reformed, but the basic system of the state demanding a certain sum to be collected from various provinces still held. Tribute in the Principate was collected usually by the agents of the quaestors and procurators, who could be just as exploitative as the publicans had been and who apparently often hired publican companies on the local level, or from the 2nd Century on conductores, who are basically the same thing but less corporate. It's important to note that the individual taxpayer in this system is largely unimportant. Provincial governors in most provinces could, and often did, raise special taxes, and the procurators or publicans raised funds more or less as they saw fit. This generally took the form of negotiation with municipal or tribal leaders, who would raise funds as necessary--sometimes this was accompanied by the threat or actual use of force. So this isn't like the IRS where as an individual you cede a certain amount of property--in practice it was probably mostly leaders, especially landlords (since many Roman taxes were in grain), who basically paid some of the produce of their lands for the community as a whole. Individuals paid tribute, sure, but not really in the same way, more through a series of middlemen than directly to the state.

Of your points, then, most don't really make that much sense under this system. Now, granted, the system of taxation in the late Empire was pretty different, and I don't know all that much about it, but the basic fact that there was no scaling income tax, which seems to be a crucial premise of your points, remains true.

Your first point doesn't make sense in a Roman legal context. The vectigals were not generally collected per person but per transaction, which means that slaves really don't make a difference. Import taxes, taxes on slave auctions, etc. aren't affected by the use of slave labor, and if we're talking about agricultural slaves, which made up the majority of slaves, then the assumption that slave labor led to a decrease in the vectigals really falls apart. See, one-tenth of the production of the ager publicus went to the state as a vectigal, and an increase of agricultural slave labor should, if anything, increase the yield of the ager publicus. The vectigals also gained importance over time, particularly after Caracalla's extension of citizenship to every free male in the Empire meant that everyone was all of a sudden paying the inheritance tax (which was the only vectigal that was paid solely by citizens).

The second point also doesn't make sense, for reasons that I've touched on. Tax rates were not based on income or existing assets, there was no scaling income tax, and it would've been pretty freaking hard to establish one for every individual. Hiding a portion of one's wealth might have helped with not getting squeezed illegally by corrupt publicans, but that doesn't matter because publicans and procurators sent fixed amounts of money in to the state and that extra squeezing was for them, not for the benefit of the public revenue.

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